AlllOl TSSIDSNBSIR 79-1939

Zinc Oxide Varistors for

Lightning Arrester Service

Robert I, Scace

I

Electron Devices Division

Center for Electronics

and Electrical Engineering

National Bureau of Standards

Washington, DC 20234

December 1979

Issued January 1980

«QC

130

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79-1939

19B0

Prepared for Division of Electric Energy Systems

Department of Energy

Washington, DC 20585

NBSIR 79-1939

ZINC OXIDE VARISTORS FOR

LIGHTNING ARRESTER SERVICE

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Robert I. Scace

Electron Devices Division

Center for Electronics

and Electrical Engineering

National Bureau of Standards

Washington, DC 20234

December 1979

Issued January 1980

Prepared for Division of Electric Energy Systems

Department of Energy

Washington, DC 20585

'a. T J

0^ U S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jorden J. Beruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

Page

Abstract 1

1 . Executive Summary and Recommendations 1

2. Introduction 2

3 . ZnO Varistors 4

4. Physical Properties of ZnO Varistors 5

4.1 Microstructure and Composition 6

4.2 Electrical Properties ..... 7

5. Models of Varistor Behavior 10

6. Application of ZnO Varistors ....'. 12

6.1 Measurement Problems in ZnO Varistor Technology 13

7. Acknowledgments 18

8. References 18

LIST OF FIGURES

Page

1. The current-voltage characteristics of a ZnO varistor at 77 K

and for a small range of temperatures near 300 K 5

2. Equivalent circuit model for the behavior of the dielectric

constant with frequency 9

3. Normalized volt-ampere characteristic of zinc oxide and sili- con carbide valve elements 13

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zinc Oxide Varistors for Lightning Arrester Service

Robert I . Scace

Electron Devices Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, DC 20234

Abstract - The application of zinc oxide (ZnO) varistors to high-power surge arresters for electrical

transmission lines is considered, with particular attention to the special measurement problems posed by

the application and by the unique properties of these

varistors. The development of ZnO varistors and the

present theory of ZnO varistor action are discussed.

Consideration is given to the need for further elaboration of the theory.

Key Words: Electrical transmission; electrically

conducting ceramics; lightning arresters; nonlinear

resistors; semiconductors; surge arresters; varistors; voltage-variable resistors; zinc oxide.

1 . Executive Summary and Recommendations

All of the companies engaged in development of lightning arresters using

zinc oxide (ZnO) varistors were visited, and the technical literature was

examined, to learn about the present state of knowledge of ZnO varistors

and about the measurement problems that might exist. It is clear that

significant problems of measurement and understanding for these varistors

abound. Enough is known to make the application of ZnO varistors to

surge arresters for transmission line service both likely and attractive.

The characteristics of ZnO varistors are sufficiently more stable than

those of silicon carbide (SiC) varistors that their use while continuously energized is possible, though much remains to be learned.

While a theory of the basic mechanisms of varistor operation has been developed, improved understanding is needed, especially of the relation between the microstructure, the energy levels arising from it, and the

electrical properties. Explanations are needed for the changes in characteristics with time and stress, and what the related structural causes

are. It is conceivable that the instabilities that are seen are not re- lated to the varistor effect but to the presence of mobile ions or other

charge-carriers, and that they can be better controlled. Without better

basic understanding, the possibility of achieving this improved control

is much reduced.

Electrical test facilities which were adequate for characterizing SiC

varistors are not sufficiently precise for evaluation of ZnO varistors.

The much-reduced leakage currents, greater degree of nonlinearity, and

generally more stable electrical properties of ZnO all combine to require

greater accuracy of measurement than heretofore. Varistors are charac1
terized at low power levels using dc, at intermediate levels using ac,

and at high power with pulsed do. The desired measurement accuracy of

0.1 percent can be achieved only with the greatest difficulty if at all.

The precise measurement of highly distorted ac currents is not possible.

Even consistency of measurement in the three power ranges mentioned is

hard to obtain.

Because ZnO varistors are intended to operate while energized continuously, there is much greater concern about the stability of electrical characteristics than was the case with SiC. In the absence of any significant service history, the relationship between life test results in the

factory and service life is unclear and unproven. The absence of a suitable model for the causes of electrical instability only sharpens this concern.

2. Introduction

A varistor is an electrical resistor whose resistance is not constant,

but is a function of some externally imposed variable. The varistors to

be described in the following report have a resistance which is strongly

dependent on the applied voltage, or electric field. The sense of the

change is such that the resistance drops with increasing voltage.

Devices of this kind have found wide use as voltage limiters for many applications. When the applied voltage rises, the resulting decrease in

the resistance of the device causes the current through it to increase

much more rapidly than Ohm's law would predict; the voltage drops in the source impedances in the circuit likewise increase, and the voltage rise

at the varistor location is held to a lower value than would otherwise be

the case. The energy of the voltage surge is dissipated as heat in the

varistor and in other resistive parts of the circuit.

There are many types of devices which are useful for voltage limiting.

They include Zener diodes, spark gaps, and varistors made from SiC or carbon blocks, as well as those made with ZnO. Table 1 is a listing of

the advantages and problems associated with these devices. It should be

recognized that each of these kinds of voltage limiters has particular

areas of application for which it excels over the others. Table 1 should

not be taken to mean that any one of these devices is superior for all

applications.

For transmission line service, the standard product for surge limiting

(called an "arrester") is an SiC varistor in series with a spark gap.

The gap is needed because the varistor has only a limited variation in

its resistance. It cannot meet the simultaneous requirements that its

conducting resistance be low enough for effective voltage limiting and

that its standby resistance be high enough that no significant heating of

the varistor occurs. The varistor is only connected to the transmission

line when a voltage surge has broken down the gap.

This device thus has two principal shortcomings. First, the gap must

fire, which requires a substantial overvoltage; further, the limited re2
Device Type

Carbon Block

Selenium

Silicon Carbide

Spark gap

Zener diode

Zinc Oxide

Table 1 - Voltage Surge Limiting Devices

Relative Strengths

Very inexpensive.

Relatively good energy

cdjsorption rating.

Inej^ensive; can have

large energy absorption

rating.

Can handle large currents; low leakage current.

Very sharply defined

onset of limiting; low

dynamic resistance;

stable characteristics.

Ine;g>ensive; can have

large energy absorption

rating; large design

range; very good a; temperature insensitive.

Limitations

Low impedance; characteristics vary with time and

temperature; rarely used

except in telephone service.

Large; costly; limited

temperature rise; significant leakage current.

Hi^ leakage current; limited resistance change with

voltage; less stable than

ZnO.

Low impedance when arcing;

difficult to interrupt;

costly in small sizes;

breakdown voltage sensitive to environment.

Limited power dissipation;

relatively e;^ensive.

Leakage is temperature dependent; some change of

characteristics under heavy

surges.

3
sistance range of SiC does not allow the varistor to clcutip the voltage as well as one would like. Both of these factors lead to the overall result

that these arresters can only hold the maximum voltage during a surge to

about 1.8 times nominal at best; twice nominal is common.

3. ZnO Varistors

Small ZnO varistors have been available for several years [1]. Electrically, they have the characteristic of two back-to-back Zener diodes.

That is, they limit overvoltages equally in either polarity. Physically,

the small devices look like ceramic disc capacitors. Structurally, the

device is a ceramic body composed of a micro-crystalline ZnO major con- stituent with an intergranular phase composed of a number of other oxides. These other oxides are termed "additives” and may include oxides

of boron, bismuth, arsenic, antimony, silicon, cobalt, titanium, manganese, chromium, etc.

Most varistors include four or more additives in mixes which are empirically arrived at and which are proprietary. While the mixture of materials is prepared by standard ceramic manufacturing techniques using "high

purity" raw materials, by semiconductor standards these are dirty products. Impurity levels in the source materials range up to 0.1 percent.

Yet the finished device is a semiconductor, and it can be characterized

by some of the same methods which are used for silicon devices.

The I-V characteristic of a ZnO varistor is shown in figure 1 for one polarity of applied voltage. In the nonlinear (breakdown) region, the cur- rent is given by the traditional empirical relation [2] a

I = kV , where a ranges typically from 25 to 60. At current densities of about

100 A/cm^, the current begins to be limited by the series resistance of

the ZnO grains, while at the other end of the characteristic, there is an excess current which has a strong temperature dependence [3] .

An ideal varistor would have no series resistance region (maintain

effective voltage limiting right up to its thermal limit), have minimal

and stable leakage current, and a high nonlinear exponent a. Typical

varistors can absorb up to 100 J/cm^ before their thermal stability

limit is reached and they must be allowed to cool.

The application of varistors of this type to high-voltage transmission

lines is appealing because they should control transient overvoltages to

a greater degree than presently available surge arresters can. It is intended that ZnO arresters should limit voltage surges to 1.5 times nominal. They should not need series gaps, so they must be able to stand being on line continuously without adverse effect for the normal 20-year

service life of such hardware.

4
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Figure 1 . The current-voltage characteristics of a ZnO varistor at 77 K

and for a small range of temperatures near 300 K. A simple extrapolation

of the breakdown characteristic at 298 K is indicated by the dashed line.

The grain resistivity limit is drawn for p =3.2 ft* cm. The coefficient

of nonlinearity, a = dlnl/dlnV versus I, is shown in the inset [31].

4. Physical Properties of ZnO Varistors

The initial development of ZnO varistors was done in Japan [2-4] . Earlier Russian work on zinc oxide includes one reference to varistor action

[5] , but there seems to have been no further development of the topic;

the Japanese work is apparently independent of the Russian [ 6 ] . Pure ZnO

and ZnO ceramics with additions of beryllium oxide or magnesium oxide are linear resistors. Additions of oxides of calcium, strontium, or barixim cause nonlinear resistive effects [4] . Varistor action is greatly en- hanced with the addition of bismuth oxide (Bi 203 ) [ 2 ] or cobalt oxide

(C03O4 ) [7]. The former is the more commonly used.

Understanding of the nonlinear resistance has been sought through studies

of the microstructure [2,3,7-16] and of the electrical properties. The

latter measurements cover the voltage drop across a single intergranular

boundary [17-23]; the dielectric constant, and loss tangent, from

10"^ to 2 X 10^ Hz [24-26]; and electrical responses to high pressures [27,28], light [29,30], and low temperatures [31,32]. Process

variables have also been ejg>lored [33,34]. Efforts to describe the behavior of ZnO varistors in terms of accepted semiconductor theory have

proceeded in parallel with the above measurements [2,17,19,21,31,35-44].

5
4.1 Microstructure and Composition

Detailed recipes for formulating the mix of oxides and processing it into

varistors remain proprietary and will unquestionably remain so. It is

likely that most of these are similar to that given by Matsuoka [3] , which calls for 0.5 mol% each of 31203 , CoO, MnO, and Cr203 ; 1.0

mol% of Sb203 ; and the balance ZnO. This mixture is referred to by

most later workers as typical. When fired at 1350*C for 1 h, this com- position has an a of 50 and a voltage gradient of 135 V/nm. Unless

otherwise specified, this formulation will be taken to be the one under

discussion in the following sections.

When the ceramic body is either sectioned or broken and examined microscopically, it is seen to be composed of small crystals of ZnO in a ma- trix of other ingredients. The ZnO crystals range in size from about 3

to 50 ym on the average; the size is influenced both by the firing

(sintering) conditions of time and temperature and by the composition of

the mix. Some additives act to limit grain growth. It has been found

that the voltage gradient is inversely related to the grain size because

the voltage drop across each intergranular boundary is essentially independent of the formulation and of the processing conditions. Hence,

there is a degree of control possible in establishing the voltage gradient of the finished varistor.

At first, it was thought that the intergranular matrix completely sur- rounded the ZnO grains, but later work has shown that this is not the case [9,11,15]. Ion scattering. Auger electron, and transmission electron microscopical analyses have all shown that over the larger part of

the boundary between ZnO grains the thickness of the intergranular layer

is at most 5 nm, if indeed there is such a layer at all. Only at threeand four-grain intersections (edges and corners) does the thickness of

the intergranular matrix vary with the total eunount of additives, and

this matrix is electrically insulating.

In a ceramic with the composition given by Matsuoka, but with tin oxide

substituted for the chromium oxide, Wong [12] has found the intergranular

material to be composed of a spinel (Zn(Zn4 y3 Sb2 /3

) 04 ) doped with Co^'*’ and Mn^'*’ and of an oxygen-deficient pyrochlore (Bi 2 Zn4 /3 Sb2 /30g)

similarly doped. The ZnO was found to be doped with Co^'*’. The proportion of spinel increased with increasing firing temperature, and a decreased. The connection between these structural changes and electrical

ones is not understood.

The electrical resistivity of the ZnO grains is in the vicinity of

1 f2 *cm, as measured by infrared reflectance [30] , rf measurements [25] , and pulsed high-current methods [25] . It does not seem to be strongly

affected by composition or process variables and Is always n-type.

Whether these characteristics arise from doping with one of the additive

ions (such as Co^"^) or from oxygen vacancies [45] is not known. Unusually strong accumulation layers (greater than 5 x 10^^ electnjns/cm^

are produced on the surface of ZnO by adsorption of oxygen [46] . Since

there is evidence that the intergranular boundary which is the site of

6
varistor action is also the site of adsorbed bismuth [11] in an unknown

state of ionization or partial oxidation, it is tempting to speculate

that varistor effects are in some way connected with chemisorbed material

which induces a strong depletion layer in the surface regions of the ZnO.

Variations in the electrical properties of ZnO varistors are correlated

with changes in the sintering conditions. From the very beginning, it

has been observed that leakage currents and a are functions of both sintering temperature and the duration of firing [2-4], with a becoming very

low with long firing times at high temperatures. Marked nonlinearity has

been observed over a range of Bi concentrations of 60:1 [9]. Bismuth has

been left out of the mix and later diffused in [3] , and excellent varistors have resulted. Under reduced oxygen partial pressures during firing, the barriers disappear and the ceramic has a linear resistance [11].

Such observations, in the absence of any good understanding of how the

physical arrangements of the different atomic species that are present

affect the nonlinear resistance, have led to much empirical work to optimize the varistor for different applications. The kinds and proportions of additives, the preparation of the mix, and sintering conditions

of time, temperature, atmosphere, and cooling rate [18] are all variables; there is a rich field of exploration for the applied alchemist.

4.2 Electrical Properties

The fact that ZnO varistors are polycrystalline bodies presents immediate

difficulties in understanding their behavior. A nximber of parallel paths

for current are available. Some of these paths traverse more grain boundaries than others; the number of boundaries has a statistical distribution. Thus, voltage measurements on the ceramic body are related to the

voltage drop across the intergranular boundary somewhat indirectly.

Direct measurements of the intergranular voltage drop have been made

[14,16,18,21-23,40]. The best value seems to be about 3.6 + 0.2 V at 1 A/cm^ at room temperature [40] . Careful probing measurements and observations by scanning electron microscopy with voltage contrast both

support the belief that the voltage drop at all but the highest currents

is confined to the neighborhood of the grain boundaries.* The temperature dependence of the voltage drop has not been measured on individual boundaries, since it is easier and just as accurate to measure it on

a large specimen. The temperature coefficient is negative and is in the

range of 2 to 5 x 10“^/®C over a temperature range of -30 to 120®C

[17,31,38]. Both the intergrain voltage and its temperature coefficient

at a fixed value of current are essentially independent of composition

and of the details of the fabrication process.

*This conclusion is supported by work using photoexcited carriers in

varistors both with and without cobalt, which changes the optical absorption behavior of the ZnO [29]

.

7
As figure 1 shows, a is not a constant for real varistors. This specimen

has an a greater than 1 0 over a range of about 1 1 orders of magnitude of

current at room temperature. At the low-current end of this range, the

onset of pre-breakdown leakage current causes the decrease in a; as the

temperature is decreased, the leakage current also drops, and high values

of a are retained at lower levels of current. At high current, a is decreased because of the series resistance of the ZnO grains themselves.

This resistance increases at low temperatures as carriers are frozen out.

Apart from these effects of leakage current and series resistance, the

magnitude of the nonlinearity coefficient is not much affected by temperature.

The leakage current has a more complex behavior. For temperatures of 0“C

and hii^er, it has an exponential dependence on reciprocal temperature

with an activation energy of 0.82 eV [38]. At lower temperatures, there

is a marked change, with long-time-constant polarization currents being

dominant. These currents have an activation energy of about 0.035 eV

over the temperature range of -196 to 102®C [26,31]. Leakage currents

increase with increasing ZnO carrier concentration [43] and have linear

dependence on the applied voltage [31]. It has been possible to separate

the polarization current component from the steady-state component and to

show that the steady-state part has essentially the same activation energy down to temperatures at which it becomes too small (<10"^^ A) to

be easily measured [26] .

The cur rent- VO Itage characteristic of a ZnO varistor can become displaced

as a result of high-current surges having a dc component, or from longtime application of steady dc fields [31]. These effects have not been

widely reported in the literature, so quantitative information is scarce.

It is possible that these polarization effects are related to the lowlevel polarization currents described above. Several workers in the

field have said privately that these effects relax with time or can be

completely removed by annealing at elevated temperatures.

Dielectric constant measurements were initially made to determine the

thickness of the intergranular layer on the assumption that the layer was insulating and that it was the factor which determined the dielectric

constant. More recent work, described above, has established that the

intergranular layer is indeed insulating, but that it is not present in

the current path between grains as a layer more than at most a few nano- meters in thickness. Therefore, the layer whose capacitance is measured

is a depletion layer.

The real part of the dielectric constant at room temperature and at a freqaency of 10 Hz is about 1500 [24,26]. This implies a depletion layer

of about 100-nm thickness. There is a gradual drop in dielectric con- stant as the frequency is increased to 10^ Hz, followed by a sharper

decrease of about 30 percent by the time that 10® Hz is reached. Above

that frequency, there is no change up to 2 x 10® Hz, beyond which there

is a sharp resonance. The dielectric constant peaks at a value above

1600 at 1.34 X 10® Hz, for the specimen studied [25].

8
6

Figure 2. Equivalent circuit model for the behavior of the dielectric

constant with frequency [25]

.

An equivalent circuit such as that in figure 2 has been used to model

this behavior [25] . A pure capacitance is in parallel with a voltagedependent resistor, ^int' series with this combination is the

ZnO grain resistance and an inductance which is an intrinsic part of

the ceramic body and not associated with the connecting leads. The

observed resonance is the series resonance of the capacitance and the

inductance. The parallel resistance has a frequency dependence which is

almost 1/f at low temperatures (-196®C) and which has a small superimposed dispersion at higher temperatures.

At frequencies above 10^ Hz, the parallel resistance is sufficiently

low that the series resistance dominates. The ZnO grain resistivity deduced from this measurement agrees with that found by infrared reflectance [25,30] and is less than the value found by high-current pulse measurements. It is postulated that the added resistance in the latter case results from the fact that not all of the interface area between grains

is equally available for passing current, but there is a spreading resistance component. If this is correct, then only about 10 percent of the

physical contact area between grains is involved in the current path.

The dissipation factor is typically low, about 0.03 or less, except for

one broad peak which rises to 0.1 at 2 x 10^ Hz at room temperature.

The frequency at which the peak occurs depends on temperature in a way

consistent with an activation energy of 0.36 eV [24] . Levinson and

9
Philipp, whose work on the dielectric constant and dissipation factor is

cited above, have concluded that this behavior is consistent with that of

many other dielectrics. A number of these are cited in reference [24]

;

all of them possess a highly disordered structure. It is not clear

whether the dissipation in the varistor is related to disorder in the depletion region or to an unrelated, different mechanism.

5. Models of Varistor Behavior

Most of the work described in the foregoing has been done to gather information which can be used to identify the mechanisms by which the non- linear resistance arises. There have been a number of models proposed,

but most of these have not been successful in explaining all of the ex- perimental evidence. The most satisfactory theory at present is based on

a double Schottky barrier model.

This model was first proposed by Levine [36] , who was able to account for

the observed dc capacitance, the departure of the current from a power

law at low voltages, and for the magnitude of the nonlinear exponent

and its modest temperature dependence. This approach has been further

developed [39-44] ; the most comprehensive treatment of the theory of ZnO

varistors is given in reference [43]

.

This reference also contains a re- view of other, previously proposed, theories and shows why they have been

found unsatisfactory or incomplete.

The model is based on well-understood semiconductor theory and explains

the following properties without the use of ad hoc constants:

- the value of the intergrain breakdown voltage and its insensitivity to temperature and ZnO donor density changes,

- the current-voltage characteristic of a single intergranular

barrier over 13 orders of magnitude in current density,

- the value of the nonlinear exponent a, and the facts that the value is larger and that the peak value moves to lower current densities at low temperatures, and

- the variation of capacitance with voltage.

With regard to the last item, the theory predicts that as the breakdown

region is reached, the capacitance should rise sharply. This had not

been previously reported in the literature, probably because the measurement of capacitance in the presence of large currents is difficult. Such

behavior in agreement with the predictions of the model has since been

found [43,44]

.

The model has other features which can be tested experimentally. It

gives quantitative relations between the leakage current and the donor

*The "upturn" region at high current densities is not included in the

model.

10
densities in the ZnO grains and similar relationships between a and the

ZnO doping. The resistivity of the ZnO can be measured by at least two

independent means, by infrared reflectance, and by microwave absorption,

which have been shown to agree reasonably well. These relationships have

not been reported in the literature, but the means to do such a study are available.

There are some important areas which the model does not address, nor was

it intended to. It makes no statement regarding the structural details

of the interface between the grains, which is assumed only to be thin and

to have a high trap density. There is no theoretical understanding of

the relationships between the electrical properties of the varistor and

the structural details of the ceramic. One cannot predict the effect of

a change in composition or processing on the electrical behavior in sufficient detail to be useful. Thus, any potential improvements in the

characteristics of a varistor which are consistent with the predictions

of the model (and thus likely) must still be obtained empirically.

This lack of understanding is of particular concern with regard to the

leakage current. As will be seen later, leakage currents and their

changes with time and electrical stress history are important in lightning arrester service. The present model tells little about the sources

of the long-term polarization currents [ 26 ] and whether or not these are related to other observed polarization effects. One would like to know

if these are the result of trapped charges or of ionic movements; the approaches to reducing these effects are likely to be different depending

on their source.

It should be pointed out that the development of a complete theory of how

ZnO varistors work and the details, at an atomic level, of how the structure of the ceramic is related to its function is a very difficult problem. The present model and its underlying theory are drawn extensively

from work on much simpler structures and materials: the monocrystalline,

high-purity elemental semiconductors such as silicon and germanium. The

complications introduced by the polycrystalline nature of ZnO varistors

are very great, and one cannot work with simpler structures because the

nonlinear resistance is a property of the intergranular interface. One

cannot expect to draw on the supply of knowledge derived from simpler ma- terials indefinitely, since intergranular effects are not dealt with

there.

One particularly important topic on which more theoretical and ejq>erimental knowledge is essential is the subject of interface states in the intergranular region. The present model, given this kind of information,

could be developed further to give more accurate predictions of performance of varistors. If, in addition, the structural origins of these

states were known, then the materials and process work needed to design

varistors with improved properties would be more likely to produce the

desired results than would be the case if only the present empirical

methods were available.

11
6. Application of ZnO Varistors

The original applications envisioned for ZnO varistors were as surge ar- resters in low-energy circuits, particularly as protective devices for

semiconductor components which are sensitive to overvoltages [1]. Presumably, this choice was made for the reasons that a large nximber of potential points of application already existed, the transition to solidstate circuits in many large-volume applications such as television was occurring, and the varistors could be made in volume and sorted out by

their electrical properties later (much as transistors are). The latter

is attractive for a product which is new and for which one is not sure of

the degree of control of its characteristics at the time of manufacture.

At the same time, there was a growing awareness of the prevalence of

voltage surges on low-voltage lines [47-49] . Residential data show a wide range of such surges, with one home ejq>eriencing a 2-kV, one-cycle

surge on the average 0.4 times an hour on a nominal 120-V circuit. The

surges originate in switching operations inside the home, out of control

of the electric utility company, as well as from lightning and line

switching on the utility's circuits. The energy content of these surges

is not well known since the source impedance is not controlled, nor was

it measured in these studies.

With the further development of the varistor and the improved understanding of its technology, a serious effort was begun to apply ZnO varistors

to higher power circuits. There are several differences between this application and those mentioned earlier. Obviously, the surge energy levels are far greater. The varistor must be designed and made to meet a well-defined specification, and it will displace an existing product (SiC

varistors)

.

One such design is described in the literature [50] . While this refer- ence contains few specifics about the physical size and other mechanical

features, there are many useful comparisons with SiC varistors and ar- resters made with them. Notwithstanding all of the problems described

above, ZnO is a much better varistor material than SiC. The latter has

an a which is an order of magnitude lower; a comparison of the I-V characteristics is given in figure 3. The leakage current of SiC is larger

and less stable with time, temperature, and electrical stress. SiC has a temperature coefficient of resistance of -0.24%/®C, compared with

-0,03%/®C for ZnO. At very high current densities, the voltage upturn

for ZnO is much less than for SiC.

Because of these characteristics of SiC, arresters using this material

must have a series spark gap. A SiC varistor which has a sufficiently

low resistance to be an effective surge limiter has too low a resistance

at nominal circuit voltages to be left on line continuously; the standby

power dissipation is too great. The spark gap has a special design characteristic which causes it to have a current-limiting action. When an overvoltage surge occurs, the gap fires, and the varistor is connected to

the circuit.

12
400

Figure 3. Normalized volt-ampere characteristic of zinc oxide and sili- con carbide valve elements [50]

.

The current- limiting action of the gap does not develop until a few hundred microseconds have passed. During that time, the varistor draws too

much current because the nonlinearity of SiC is insufficient and because

the gap has not developed its current- limiting action. In cases where

the source impedance of the circuit is particularly low (in the vicinity

of capacitor banks, for example), the gap may not develop any currentlimiting effect and can be damaged by the high current. The spark gap

assembly has a number of components which make it costly, and it is sensitive to contamination which can change its sparkover voltage.

For these reasons, it would be desirable to have a surge arrester which

did not require any spark gaps. In such an arrester, the varistor is

connected to the circuit at all times and must be able to maintain its

characteristics, especially in the leakage current region, over a long

period without significant change. This is a very important difference;

in spite of the large improvements in properties of ZnO varistors in com parison with SiC ones, it is not a trivial task to apply ZnO to this application. The work reported [50] is the result of over five years' effort, and at the time of its publication, only one field installation,

of an experimental nature, was reported to have been made.

6.1 Measurement Problems in ZnO Varistor Technology

In the discussion which follows, much of the subject matter was learned

in visits to the companies that are developing ZnO for surge arrester

service. No association of a given topic or problem with a particular

firm will be made. These subjects are not described in the literature,

due to the fact that they are unsolved problems or are felt to have a proprietary aspect. The discussion will follow the general sequence for

manufactxire and evaluation of the varistor.

The raw materials, such as ZnO, 31203 , and the like, in laboratory

work are usually of reagent grade. It is desirable to keep the uncontrolled variables to a minimum, and thus one tries to use the highest

purity source materials. In production, cost is important. Lesser qual

ity grades of materials cost less, and there is great economic value in

13
knowing what specific impurities are likely to cause problems and which

are not.

To this end, most companies are using analytical methods such as emission

spectrography, atomic absorption, and similar techniques to analyze the

input materials. One then tries to correlate the results with the properties of the varistor. The sensitivity of most of these methods is in

the neighborhood of one part per million, depending to a great extent on the specific element in question. There is no assurance that this level

of detection is sufficient.

Furthermore, to calibrate the analytical method, it is usually necessary

to prepare a reference standard of the matrix material, such as ZnO, with

known amounts of the probable impurities. A sample of this standard is

analyzed at the same time or just before the unknown specimen is tested,

and a comparison of the results allows removal of most sources of error. Unless they have been made in-house, standards are not available for this

calibration. Accurate assay of the standard requires evaluation by an independent means with a greater detection sensitivity than the method

that is normally used for analysis. Such a means is not usually at hand, or it would be used for the evaluation of the specimen. That fact, and

the considerable time and skill required for preparing and assaying of

standards, usually means that routine analyses of the materials used to

make varistors are not done with great accuracy.

Undesired, and usually uncontrolled, impurities can enter the process

from any of the raw materials, during ball-milling and other blending

operations, and from solvents or binder fluids used in preparing pressed

bodies for firing in the kiln. An approach to control of these impurities by purely analytical methods is both time-consuming and troubled by

the inaccuracies mentioned above. Most firms instead use a comparative

method of control, in which a supply of known good materials (as determined empirically) is set aside to be used as controls in the evaluation

of newly acquired supplies. The new material Ccin thus be qualified for

use if it produces varistors as good as those made from the known control

material.

Little new information is learned from this way of operating. With care, one can be protected from serious yield losses if a lot of raw material

is not suitable because this method of testing will usually uncover that

fact early. The specific reason why a bad lot is bad is not learned, and

the opportunity to improve the capabilities of the suppliers of raw materials in producing better materials is lost.

There have been cases involving additives in which all of the additive

materials Independently passed such a comparative qualification test but

In combination did not produce good varistors. Each of these instances

was eventually found to involve a pair of the additive materials. Either

member would qualify singly, but they would fail in combination. The

real cause for this problem. In terms of kinds and amounts of undeslred

Impurities or In any other sense. Is not known.

14
Another problem with the empirical approach to control is related to the

decision of what is "good" when the varistors are evaluated. Not all

problems in varistors show up immediately, but they appear after some time on life test. In practice, there is only a limited time for life

testing when materials acceptance cjuestions must be answered. If material is to be rejected, only a limited time is available before the vendor

will refuse to accept its return for credit; decisions must be made soon- er than that, whether or not life testing has provided the information

needed to make the decision to accept or reject the material.

In one way, it is possible to combine the empirical and analytical approaches to control to yield some objective information. If a lot of ma- terial fails, it can be analyzed in comparison with the control material.

An absolute calibration may be lacking, but at least some relative, seraiquantitative information is obtained about the impurities present in the

two specimens.

After the varistor has been fired in a kiln and transformed into a ceramic, evaluations are mostly electrical. However, measurements of grain

size and other gross structural features are done with both optical and

scanning electron microscopy, and samples are scanned ultrasonically for

the presence of cracks or voids, which will affect the electrical and

thermal properties of the varistor. There do not seem to be any serious

measurement problems in these nonelectrical areas.

There are, however, difficulties with the electrical measurements. Most

companies measure leakage currents by dc methods. The currents are small

enouc^ that no heating of any consequence occurs during the test, an important consideration in this current range because of the strong temperature dependence of the leakage. Pulsed dc methods are used up to the

maximum currents of 10 kA or so. Some firms use sine-wave ac tests at

the lower end of the nonlinear range. Most* do life tests under ac excitation.

Because of the very wide range of currents which must be measured, from

microamperes to kiloamperes, and because of the corresponding range of

power required from the test equipment, no single piece of equipment can be used over the whole range. This is further complicated by the fact

that the tests use ac, dc, and pulse methods. Consistency of calibration

is a problem, with most firms trying to achieve 0.1-percent accuracy.

This is felt to be needed because the value of the nonlinear exponent is a sensitive measure of certain processing parameters. High accuracy is

required in the voltage measurements to allow the e;q>onent to be calculated with useful precision.

Correlation of measurements over the whole voltage range, using the various excitation methods mentioned above, is made more difficult by the

Possibly all do, but confirmation of this is lacking from one company.

15
fact that the varistors do not respond in the same way to dc as they do

to ac. Polarization currents which are apparent in dc tests but which

have time constants of the order of and longer than a half-period of the

60 Hz ac (8.3 ms) do not show up in ac measurements. Furthermore, in ac measurements most of the current at low levels is capacitive; the inphase (dissipative) component is small and difficult to measure accurately. This is just the current component which relates to the dc leakage

measurements. In ac tests at higher levels, the current has a marked

peak at the crest of the voltage. This distorted current waveform can have a crest factor which is larger than the rating of rms-to-dc converters used in the measurement and which leads to measurement errors of un- known amounts.

Measurements of 0.1-percent accuracy cannot be done oscillographically

.

Digital instruments are required. These instruments respond to transients and there is concern that correct steady-state measurements are not obtained. Most digital voltmeters have a sample-and-hold circuit at

the input which follows the signal until the measurement command is re- ceived. At that time, the value of the signal is held and measured.

If the value which is held is a transient one, the measurement will be in error. Transients are particularly troublesome when pulse measurements

are being made, because at high currents very large rates of change exist

and appreciable voltages are developed across small inductances.

High-power measurements are usually made with an 8 by 20 wave, that is, one with a rise time of 8 ys and which decays to half of the peak value

in 20 ys. In a linear resistor, a 10-kA pulse of this form would have a rate of rise of about 1.25 x 10^ A/ys, but in such nonlinear resistors

as ZnO varistors, the application of an 8 by 20 voltage pulse of appropriate magnitude to develop 1 0 kA of current results in rates of current

rise much larger than that. This exacerbates the transient problem.

Some workers have tried using filters to avoid having the measurement instruments respond to transients, but there is concern that the filters

may introduce other errors. There is no industry standard which deals

with this problem.

Other difficulties exist in connection with life testing. The application of ZnO to high-power transmission lines is new and differs significantly from previous practice. To gain acceptance by the electric utilities, manufacturers of arresters using ZnO must be able to give assurance

that the product will serve reliably for 20 years or more. Accelerated

life tests are necessary, since 20 years are not available for testing.

The only feasible acceleration factor is temperature. Electrical over- stress is not feasible, since the current increases so rapidly with voltage; if this approach were tried, the temperature would increase markedly

as well, and one would have the difficult problem of separating the effects of the thermal and the electrical acceleration factors from one another. ZnO varistors have been observed to develop asymmetrical electrical characteristics with prolonged dc stress or after large surges,

with the effects being reversible after annealing at moderate tempera16
tares. There is some concern that life testing at elevated temperatures

may not be entirely representative of service applications on a com- pressed time scale, and that some annealing of possible instabilities or

of changes in characteristics may occur during the test.

This question is reminiscent of those raised in the early days of the application of high-power silicon devices to similar service. There is

only one way in which such issues can be resolved without question. A

history of performance must be developed which can be compared with the

results of accelerated tests to show to what degree the latter can be depended upon as indicators of service life. Carefully chosen pilot installations, with extra instrumentation to record the events which occur

as a function of time, and a conservative design approach to minimize the

adverse effects which can result from inexperience, are helpful in gaining the needed experience without undue risk. This question will not be

solved overnight.

There has been some difficulty encountered in obtaining stable, lowimpedance ac sources for life testing. The source must be well regulated

and have few, if any, voltage surges. One firm has observed discontinuous changes in the characteristics of varistors on life test when the

test is interrupted either accidentally or purposely. The cause of the

changes is not known.

Moreover, there are no generally accepted definitions for "stability" of

ZnO varistors, no established levels of performance with regard to stability, and no real understanding of the causes for lack of stability.

If a varistor exhibits a change in characteristics in one hour, it is re- garded as unstable, yet one firm wishes for a test that would tell in a few seconds whether a varistor is stable or not. Work is needed to develop suitable criteria and tests for stability beyond the requirements

of any one company. Both manufacturers and users of varistors need to be

involved in that work.

Another concept in need of definition and understanding is that of wearout. Small varistors are rated to absorb given amounts of energy and re- main within their guaranteed specifications. The amount of energy is not

a constant, but depends on the magnitude of the largest surges which are applied. It is not clear how this characteristic will be defined and

specified for larger varistors. The mechanism of wear-out is not understood, nor whether it is related to polarization characteristics. It has

also been observed that when a varistor is first energized on ac, the

current is higher for the first few cycles and then settles to a value

about half of the original one. This effect and its relation to those

just mentioned is not understood.

17
7.

Acknowledgments

The author wishes to thank representatives of the following organizations

for their courtesy and helpful discussions during visits to learn about

the problems of ZnO varistors;

Electric Power Research Institute

General Electric Company

McGraw-Edison Company

The Ohio Brass Company

Westinghouse Electric Corporation

Permission to reproduce certain figures in this report has been granted

by the American Institute of Physics (fig. 1) and the Institute of Electrical and Electronics Engineers (fig. 3).

8.

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21
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